Recently, in electronic integrated circuits, operation at a lower voltage, an increase in the operation frequency, and a reduction in noise have been desired. In solid electrolyte capacitors, demands for reducing equivalent series resistance (ESR) and for reducing equivalent series inductance (ESL) have been increasing. Examples of a metal powder that is preferably used as an anode material of a solid electrolyte capacitor include powders composed of tantalum, niobium, titanium, tungsten, or molybdenum. Among these, in particular, capacitors including tantalum have become rapidly widespread as components of cell phones, personal computers, and the like because of their low ESR and high capacitance. Recently, a further increase in the capacitance and a further decrease in ESR of capacitors have been desired. In order to further increase the capacitance of a capacitor, tantalum powders having a large specific surface area and a fine particle size have been developed as an anode material of the capacitor. However, when the specific surface area of the powder is increased, the oxygen content of the powder is also increased. Consequently, a crystalline oxide is easily produced during heat treatment and anodic oxidation, resulting in a problem of an increase in leakage current. When the rated voltage of a capacitor is set to a low value, the anodic oxidation voltage at which a dielectric oxide film is formed can be decreased. However, when the anodic oxidation voltage is decreased, the thickness of the resulting dielectric oxide film tends to be decreased, resulting in an increase in leakage current. Thus, such a capacitor is disadvantageous in that a long-term reliability is not satisfactory. In order to suppress such an effect of oxygen and to improve the reliability of a thin film, a method of doping a non-oxidizing gas or various types of element is known.
For example, Japanese Unexamined Patent Application Publication Nos. 2001-223141, 2001-345238, 2002-30301, and 2003-55702 describe techniques for doping nitrogen into a tantalum powder.
According to Japanese Unexamined Patent Application Publication No. 2001-223141, an anode having a high capacitance and a low leakage current can be produced by adding 50 to 20,000 ppm of nitrogen to tantalum in the form of a solid solution.
According to Japanese Unexamined Patent Application Publication No. 2001-345238, 50 to 10,000 ppm of nitrogen is contained in tantalum, thereby decreasing the shrinkage percentage in sintering at high temperatures. As a result, a capacitor including an anode composed of a sintered body of this powder easily has a high capacitance and a low leakage current. Thus, a capacitor having high reliability can be produced.
According to a technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-30301, 500 to 30,000 ppm of nitrogen is contained in a tantalum powder, and the variation in the nitrogen content between tantalum particles is controlled to 100% or less. When this material is sintered, an excessive sintering can be suppressed to obtain a porous sintered body that evenly includes voids having a size suitable for forming a solid electrolyte. The patent document discloses that the resulting porous sintered body is most suitable for a high capacitance voltage (CV) capacitor.
According to an example disclosed in Japanese Unexamined Patent Application Publication No. 2003-55702, nitrogen is contained in a tantalum powder having a specific surface area in the range of 1.0 to 4.0 (m2/g) such that a value obtained by dividing the nitrogen content (ppm) by the specific surface area is in the range of 500 to 3,000. This patent document describes that when this tantalum powder is used as an anode material of a solid electrolyte capacitor, a solid electrolyte capacitor having a high capacitance, a low leakage current, and excellent long-term reliability can be obtained.
Regarding doping of other elements, Japanese Unexamined Patent Application Publication No. 2003-178935 discloses doping of phosphorus, Japanese Unexamined Patent Application Publication No. 2002-173371 discloses doping of zirconia, carbon, boron, or sulfur, and Japanese Patent No. 2632985 discloses doping of titanium, zirconium, or hafnium.
A tantalum powder used for an anode of a solid electrolyte capacitor is prepared by performing anodic oxidation of a tantalum powder in an electrolytic solution to form a dielectric oxide film on the surface of the powder. Accordingly, it is known that the capacitance correlates with the specific surface area of the tantalum powder, and that a large specific surface area can provide a solid electrolyte capacitor having higher capacitance. However, in the case where the tantalum powder has a large specific surface area, even when nitrogen or another element is doped using the above-described doping techniques according to the descriptions of Japanese Unexamined Patent Application Publication Nos. 2001-223141, 2001-345238, 2002-30301, 2003-55702, 2003-178935, and 2002-173371, and Japanese Patent No. 2632985, a solid electrolyte capacitor having a high capacitance and low leakage current is not always obtained. The present inventors have found that, in particular, when the specific surface area of the tantalum powder is 4 m2/g or more, the above tendency is significant.
Accordingly, an object of the present invention is to provide a tantalum powder having a large specific area wherein when the powder is used as an anode of a solid electrolyte capacitor, a tantalum solid electrolyte capacitor having a large capacitance and a low leakage current can be obtained. Furthermore, another object of the present invention is to provide an anode for a solid electrolyte capacitor produced by sintering the tantalum powder and a solid electrolyte capacitor including the anode.